The present invention relates to ladder frame assemblies for motor vehicles and a method for forming the same.
In the field of motor vehicles, especially off road vehicles, pick-up trucks and similar vehicles, it has been known to provide a vehicle ladder frame assembly for mounting various components of the vehicle, such as the body, engine, suspension system, etc. Typically, the frame assembly includes a pair of lengthwise parallel siderails generally extending along opposite sides of the vehicle, and a plurality of transverse cross members interconnecting the siderails at spaced positions therealong. The frame assembly acts as the main load carrying member for rigidity and strength, as opposed to the body members for the "unibody" construction typically used in standard passenger cars.
In more recent years, vehicle frame assemblies have been manufactured with added modularity. More specifically, the vehicle frame assemblies may be conveniently divided into a forward frame module, central frame module, and a rearward frame module. This facilitates manufacture and permits certain frame modules to be used with more than one type or model vehicle. Moreover, modular design has the advantage that different portions of the frame assembly can be manufactured with different constructions. For example the forward frame module is preferably manufactured with closed bar siderails or rail members (i.e., having a closed transverse cross-sectional configuration) to provide the frame assembly with maximized strength for crashworthiness and torsional rigidity under the cab. The central and rear frame modules, on the other hand, are preferably manufactured with siderails having a U-shaped transverse cross sectional configuration, with the opened portion of the U-shaped configuration facing inwardly towards the center portions of the vehicle for better bending efficiency, of the frame assembly and to facilitate mounting of the cross members and other components. As used herein, the term U-shaped may be synonymous with a "C-shaped" configuration, wherein the term "C-shaped" refers to a more specific U-shaped configuration in which inwardly facing flanges are provided at the opposite ends of the U-shaped configuration. In addition, as used herein, the term "rail member" is interchangeable with the term "siderail".
In producing vehicle frame assemblies, manufacturers typically employ a stamping technique. Stamping is accomplished by utilizing one or more opposing stamping die members which compress (by impact) a sheet of raw metal, usually steel, therebetween to form siderail frame members and cross members of a desired shape (e.g., having the U-shaped cross sectional configuration). During the stamping process, the raw material is sequentially moved and stamped in different stamping die stations until the final shape is achieved. While stamping operations are advantageous in certain respects, the stamping dies and equipment are relatively expensive and must be replaced on a frequent basis. As a result, it can be appreciated that material, labor and equipment costs associated with manufacturing and assembly of stamped frame members is significant.
Another problem associated with stamping a straight metal sheet is that a significant amount of raw material must be wasted, especially where irregularly shaped frame parts are required. More specifically, where irregular shapes are to be stamped from a sheet of raw metal, there is a significant amount of peripheral material (particularly at the side edges of the sheet) that cannot be used, irrespective of how closely the stamped shapes are nested on the sheet. In one analysis, it was found that up to 30%-40% of a roll of raw material could not be used due to the irregular shape of the stamped part. It can be appreciated that, at high volumes, the amount of material wasted, and costs associated therewith, is significant in the stamping method.
Another problem with stamping results from the assembly of the stamped members into a frame assembly. This assembly is typically done using a wire weld process that applies significant heat to the parts. During the assembly process, the stamped parts must be clamped into position to control the tendency of the stamped parts to spring back towards their original shape and to control the stresses that are created during the heating and the forming processes. Without clamping, these stresses might otherwise result in significant distortion of the assembly (6-12 mm overall are typical). Thus, the assembly and tooling at high volumes results in high investment, manufacturing costs and rework costs to obtain a quality assembly.
The patent literature has proposed a procedure in which a siderail frame member is roll-formed rather than stamped. More specifically, in U.S. Pat. No. 2,127,618, there is proposed an apparatus which passes a sheet of raw metal material through a series of cooperative rollers which sequentially bend the sheet into a tubular siderail member having a box-shaped transverse cross sectional configuration. Roll forming offers numerous advantages over stamping. For example, capital expenditures required for manufacturing equipment is significantly lower. In addition, since roll forming machines function without the stresses of high impact associated with stamping dies, they require less maintenance and have a longer useful life. Furthermore, since a sheet of material can be continuously passed through the series of rollers, frame siderail production times are faster. Additionally, since a sheet or strip of raw material is completely used in making a siderail, little or no raw material is wasted. Assembly is minimized as the formed members are rolled during the rolling process into a complete closed box section. It can also be appreciated, therefore, that manufacturing costs associated with roll forming is significantly less than with stamping.
The benefits of roll forming, however, have heretofore been limited by the process' inability to form siderails of any desired shape. For example, in the most desirable configuration, it is advantageous not only to provide the central frame module siderail members with a U-shaped cross sectional configuration as aforesaid, but it is also desirable to provide the central portion of the siderail with bends in vertical direction, for example, to accommodate axle mountings for the rear wheels. Since, unlike stamping, the roll forming process can form only a straight siderail member (e.g., without any bending in the vertical direction), any vertical bends must be accomplished in a subsequent operation. For example, the aforementioned U.S. Pat. No. '618 patent contemplates that after formation, the straight box sectioned siderail can be bent in a bending mill to provide forward and rear kickup. While it is relatively simple to bend box-shaped cross sectional configuration because of its inherent strength, heretofore attempts at bending a roll-formed siderail member having a U-shaped configuration have been unsuccessful. More specifically, such attempts have been ineffective due to the relative weakness of an opened section configuration and its susceptibility to unwanted twisting and distortion. The use of a roll formed, closed box tubular cross-sectioned side frame member for the front rail has also been commercially omitted due to the necessity of the roll forming operations to form frame members having a constant cross section. This is impossible to package due to geometry requirements of the engine, front suspension, body and other engine compartment components. Resulting designs have not been efficient for material use and have been used only in low volume applications and large trucks, where the investment savings can justify the material cost penalty and packaging constraints are not as severe.
There has thus been a need to manufacture a frame assembly for an automobile that addresses the concerns noted above. It is therefore an object of the present invention to meet this need. In accordance with the principles of the present invention this objective is achieved by providing a method for manufacturing a ladder frame assembly for a motor vehicle. Two of the steps of the method of the present invention comprise forming a rearward frame rail module and forming a forward frame rail module. Another step is forming a central frame rail module comprising a pair of central frame siderails, each of the central frame siderails being formed by 1) moving a sheet of metallic material through a series of cooperating rollers constructed and arranged to rollingly engage opposite sides of the sheet in a manner which bends the sheet into a configuration having a generally U-shaped transverse cross section including a vertical base wall portion and a pair of outwardly extending horizontal wall portions extending from opposite ends of the base wall portion, and 2) bending the U-shaped configuration of the sheet including bending the base wall portion generally within its plane while stretching one of the horizontal wall portions extending from one end of the base wall portion and compressing an opposite one of the horizontal wall portions extending from an opposite end of the base wall portion a final step of the method is rigidly securing the front frame rail module and the rearward frame rail module to opposite ends of the central frame rail module.
The forward frame rail module preferably comprises a pair of front frame siderails which have a closed cross-sectional configuration. Where closed box frame parts are to be manufactured (e.g., at forward portions of the frame which absorb crash energy), stamping becomes even more problematic since a single integrally formed tubular member cannot be stamped. Instead, where tubular frame parts are required, it is necessary to stamp a plurality of elongated sections which must be longitudinally welded. For example, two facing U-shaped sections may be welded into a box-like sectional configuration. Such a construction is not as strong or dimensionally accurate as one in which an integrally formed tubular member is employed. It can also be appreciated that this stamping and welding process requires even further resources, such as welding material and additional man-hours to complete the weld.
It should be noted that roll forming can be used to manufacture a tubular member for the forward portion of the siderail. For example, to form the box-shaped configuration noted in the aforementioned U.S. Pat. No. '618 patent, it is necessary to roll the side edges of the strip material into the squared cross sectional shape until the side edges of the strip material are brought into abutting relation and then welded. More recently, however, it has been proposed to integrally form a closed section tubular vehicle frame member from a tubular blank in what is known as a hydroforming process. See, example, U.S. Pat. Nos. 5,107,693, 5,233,854, 5,333,775, 4,567,743, 5,070,717, 5,239,852 and 5,339,667. While hydroformed frame members are highly advantageous for the forward portions of the siderail frame assembly, heretofore, the related art has not provided a modular motor vehicle frame assembly having hydroformed forward frame rail members, or siderails and roll formed and bent U-shaped central frame rail members.
In accordance with the principles of the present invention each of the front frame siderails are preferredly formed by 1) placing a metallic tubular member into a cavity of a die mold, the die mold having an interior surface defining a shape of the cavity, and 2) providing a fluid interiorly to the metallic tubular member with sufficient pressure so as to expand the tubular member outwardly into engagement with the interior surface of the die mold to substantially conform the tubular member to the shape of the cavity.
The hydroforming procedure for forming the front frame rail module of the present invention utilizes a pair of tubular blanks that are formed into the front frame siderails. Fluid pressure is applied to the interior of each blank to expand the blank while the ends of the blank are pushed inwardly to obtain material flow so that the wall thickness of the expanded tube is maintained within .+-.10% of the original blank. The original blank is formed b a roll forming and welding operation.
The present invention also achieves its objective by providing a rail frame assembly for a motor vehicle comprising a rearward frame module having a pair of generally parallel rearward frame rail members interconnected by a rearward cross-member assembly; a central frame module comprising a pair of generally parallel central frame rail members interconnected by a central cross-member assembly, the central frame members each having a generally U-shaped transverse cross-sectional configuration including a vertical base wall portion and a pair of outwardly extending generally parallel horizontal wall portions extending from opposite ends of the base wall portion, the U-shaped central frame members each being bent between two spaced positions thereof, and one of the horizontal wall portions extending from one end of the base wall portion having metallic grains therein substantially stretched at a location between the spaced positions and an opposite one of the horizontal wall portions extending from an opposite end of the base wall portion having metallic grains therein substantially compressed at the location between the spaced positions; and a forward frame module comprising a pair of generally parallel tubular forward frame rail members interconnected by a forward cross-member assembly.